"I've seen a few of these anniversaries here but the security this time is just amazing. You can't get out of a car anywhere near the square. I'm talking like for kilometres around the square without seeing police, without seeing the People's Armed Police," he said.

"When we drove around there last night, we were photographed, followed. Just anybody who looks they're even having a look at the square is being monitored."

Despite the crackdown in Beijing, McDonell said hundreds of thousands are expected to turn out for a candlelight vigil in Hong Kong.

"They're expecting a record turnout - I mean, seriously, they're talking hundreds of thousands of people for the candlelight vigil in Hong Kong," he said.

"But elsewhere on mainland China there'll be absolutely nothing - nothing on television, nothing on the radio, nothing in the newspapers, no spin from government, not a mention from anybody in public life about this anniversary - just a very big void."

United States urges China to free activists

Many professors, lawyers, journalists and church leaders have been detained for fear they may speak out. So have artists, including Australian Guo Jian.

"We've very clearly called on the Chinese authorities to release all the activists, journalists and lawyers who have been detained ahead of the 25th anniversary," Ms Harf told reporters.

"I think it's time to allow some more space, quite frankly, for discussion in their own country, particularly around this kind of anniversary," she added.

A monitoring service said several Google websites have also been blocked in China as authorities stepped up arrests and censorship before the anniversary.

After initially tolerating the student-led demonstrations in the spring of 1989, the Communist Party sent in troops to crush a rare display of public defiance.

Stunned by the government's harsh response to the Tiananmen movement that officials have termed "counter-revolutionary", and tired of decades of turmoil under Communist rule, many Chinese people now balk at the idea of mass revolution.

Instead, they chase new opportunities offered by the country's booming economic growth.

And while the authorities have moved swiftly to squash criticism of the one-party system, people are enjoying the kind of individual freedoms never accorded them before.

They can report on corrupt officials, sue the government for pollution and miscarriages of justice, and stage protests for labour and environmental rights.

The Chinese government has also loosened the one-child policy, allowing many urban couples to have two children.

It has been effective, too, in scrubbing out memories of the 1989 protests. Many young people, indoctrinated by years of "patriotic education", have no inkling of the movement.

Beijing has forced many of the student leaders into exile in the United States, Hong Kong and Taiwan, where they are effectively neutralised, being barred from the mainland.

"Once we leave China, we've left the battlefield," said Wu'er Kaixi, a leading figure in the pro-democracy movement who now lives in Taiwan. "We are no longer the main actors on the stage."

Wang Dan, who was one of the most visible leaders in the movement and is also in exile in Taiwan, said he was able to hold a "democracy salon" - an open forum for intellectuals to discuss political problems - at Peking University 25 years ago.

"Everyone knows that anyone who dares to do anything like that these days will be detained. This is a clear regression from where we were back then."